Pepper plants are very yellow leaves on pepper plants and resilient, but their leaves turning yellow is not a sign of good health. It can indicate that the plant is stressed by water or nutrient deficiencies, disease, or cold weather. Unless you correct the underlying issue, the pepper plant will not turn back to green; however, it may continue growing and fruiting. If the disease is too serious, it is likely best to pull out the entire plant and not plant a nightshade vegetable in that location again next year.
A common cause of yellowing on pepper plants is insufficient moisture, usually because of poor drainage or a soil that is too compacted to allow the plant to soak up enough water. The solution is to loosen the soil and amend with compost or well-draining potting mix. Overwatering can also lead to a yellowish hue, so be sure that you water within a certain range.
Other causes of yellowing include nutrient deficiencies such as nitrogen deficiency or potassium deficiency. This can be caused by a lack of organic material in the soil, nutrient leaching from overwatering or rain, and an inadequate fertilization schedule. Adding a natural fertilizer with an adequate amount of nitrogen (such as this blood meal from Down To Earth) should help correct this issue.
Other problems can be more difficult to diagnose and treat, including fungal or bacterial disease. If you see dark brown spots, wilting, or yellowing on the top of a pepper plant, this is a likely indicator that the plant has a fungus or bacterial disease such as phytophthora blight, bacterial leaf spot, Southern blight, or fusarium wilt. These diseases are fatal to peppers, so it is important to rip out all of the affected plants and not plant any nightshade vegetables in that area again next year.